Power cable, why can't I choose aluminum cable?
A steel mill used aluminum alloy cables in batches. Two fires occurred in one year and the production was suspended for half a month. The direct economic loss was 200 million yuan.
This is the cable tray that has been repaired after the fire, and the fire marks are still eye-catching.
An aluminum alloy cable was used in a lighting distribution system in a certain city in Hunan. During the year after installation, strong corrosion of the aluminum alloy cable occurred, resulting in damage to the cable joint and conductor, and the line was de-energized.
Through these two cases, we can see that the large-scale promotion of aluminum alloy cables in Chinese cities, factories and mines has left hidden dangers for cities, factories and mines. Users lack understanding of the basic performance of aluminum alloy cables, and thus suffered huge losses, if users prematurely Knowing the characteristics of aluminum alloy cables in terms of fire reliability and corrosion protection, such losses can be avoided in advance.
From the characteristics of aluminum alloy cables, aluminum alloy cables have natural defects in fire prevention and corrosion protection. Expressed in the following eight aspects:
Corrosion resistance, 8000 series aluminum alloy is not as good as ordinary aluminum
GB/T19292.2-2003 standard Table 1 Note 4: The corrosion resistance of aluminum alloy is worse than that of ordinary aluminum. This is because aluminum alloy cable is added with magnesium, copper, zinc and iron, so it is prone to local corrosion. Such as stress corrosion cracking, layer erosion, intergranular corrosion, and 8000 series aluminum alloy is easy to corrode formula, aluminum alloy cable increases the heat treatment process, easy to cause uneven physical state, more susceptible to corrosion than aluminum cable. At present, the aluminum alloys applied in China are basically 8000 aluminum alloy series.
Temperature resistance, aluminum alloy is much different than copper
Copper has a melting point of 1080 ° C and aluminum and aluminum alloys have a melting point of 660 ° C, so copper conductors are a better choice for fire resistant cables. Now some aluminum alloy cable manufacturers claim to be able to produce fire-resistant aluminum alloy cables and pass the relevant national standards test, but there is no difference between aluminum alloy cables and aluminum cables. If it is in the fire center (above), the temperature is higher than the melting point of aluminum alloy and aluminum cables. When the cable is insulated, the cable will melt in a short period of time and lose its conductive function. Therefore, aluminum and aluminum alloys should not be used as fire-resistant cable conductors, nor should they be used in densely populated urban distribution networks and buildings. Used in factories and mines.
The thermal expansion coefficient of aluminum alloy is much higher than that of copper, and AA8030 aluminum alloy is higher than aluminum.
10X6/°C | copper | aluminum | Aluminum alloy AA1000 | Aluminum alloy AA1350 | Aluminum alloy AA8030 |
0-100 ° C | 17.0 | 23.5 | 21.9-25.5 | 21.8-25.5 | 23.6 |
As can be seen from the table, the thermal expansion coefficient of aluminum is much higher than that of copper, and the aluminum alloys AA1000 and AA1350 have a little improvement, while AA8030 is even higher than aluminum. A high coefficient of thermal expansion can lead to poor contact and thermal cycle of the conductor after thermal expansion and contraction, and the power supply always has a peak-to-valley difference, which has brought great challenges to cable performance.
Aluminum alloy does not solve the problem of aluminum oxidation
Aluminum or aluminum exposed to the atmosphere quickly forms a hard, strong but brittle, film of approximately 10 nm thickness with high electrical resistivity, and its hardness and adhesion make it difficult to form conductive contacts. This is the reason why the surface oxide layer must be removed before the aluminum and aluminum alloys are installed. The copper surface is also oxidized, but the oxide layer is soft and easily breaks down into a semiconductor when subjected to force, forming a metal-metal contact.
Aluminum alloy cables have improved in terms of stress relaxation and creep resistance but far less than copper
The addition of specific elements to the general aluminum can improve the creep properties of the general aluminum, but the degree of improvement is also very limited compared with the general aluminum, and there is still a huge gap compared with copper. Moreover, whether the aluminum alloy cable can really improve the creep resistance is closely related to the process, technology and quality control level of each enterprise. This uncertainty is itself a risk factor. If there is no strict control of the mature technology, the aluminum alloy cable improves the creep. Variable performance is also not guaranteed.
Aluminum alloy cable does not solve the connection reliability problem of aluminum
There are five factors that affect the reliability of aluminum connection. The aluminum alloy has only improved on one problem, and it has not solved the problem of the connection of aluminum.
There are five problems in the connection problem between the aluminum and the aluminum alloy. The 8000 series aluminum alloy has only improved in terms of creep and stress relaxation, and there is no improvement in other aspects. Therefore, the connection problem will still be a major problem affecting the quality of the aluminum alloy. Aluminum alloy is also a kind of aluminum and not a new material. If the basic performance of aluminum and the difference of copper are not solved in terms of conductor performance, aluminum alloy cannot replace copper.
Inconsistent quality control of domestic aluminum alloys (alloy composition) results in poor creep resistance
After the Canadian POWERTECH test, the composition of domestic aluminum alloy is unstable, and the Si content of aluminum alloy cables in North America differs by less than 5%, while the Si content of domestic aluminum alloys differs by 68%, and Si is an important element affecting creep performance. That is to say, the domestic aluminum alloy cable has not formed a mature process in terms of creep resistance.
Aluminum alloy cable joints are complex in process and easy to keep hidden
The aluminum alloy cable joint process has three more processes than the copper cable. The effective removal of the oxide layer and the application of the antioxidant are the key points. The domestic construction level and quality requirements are uneven, leaving hidden dangers. Moreover, since there is no strict legal liability compensation system in China, the consequences of the final loss are basically borne by the users themselves.
In addition to the above factors, there is no uniform standard for the interception flow of aluminum alloy cables, but the joint terminals are not closed, the capacitance current is increased, the spacing of the aluminum alloy cables is narrowed or insufficient to support, and the cable cross section is increased, resulting in construction difficulties. Whether the space of the ditch # space, maintenance and risk costs rise sharply, the life cycle cost increases, the designer can follow a series of professional issues, such as improper handling or deliberately neglecting any one of them is enough for users to suffer Heavy losses and accidents that cannot be repaired.
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